The tunnel boring machine advancements in horizontal
and up-hill tunnels.

Abstract:

Problem statement: Nowadays high levels of experience have been
acquired in the excavation of horizontal tunnels using TBM, especially
as far as tunnels with small diameters (about 4 m wide) are concerned.
Less experience has been acquired in the excavation of tunnels under
difficult alignment conditions, as in the case of steeply inclined
excavations (up-hill tunnels). Approach: This study presented the
results of studies which compare the production data collected for
tunnels excavated with TBM in "normal" conditions (horizontal
tunnel), with those derived from steeply inclined excavations (up-hill
tunnels). Results: From an examination of the results obtained in the
studied cases a net difference appears evident in terms of productivity
between horizontal and up-hill excavations. Conclusion: Such net
difference between horizontal and up-hill excavation productions can be
attributed to the greater burdens of times necessary for the supply of
materials and personnel, the regripping operations with fall preventer
systems and, in general, the greater difficulties of carrying out
ordinary and extraordinary maintenance of the machines in particularly
difficult altrimetric conditions.

As known, the productivity of an excavation system with full
section machines, called Tunnel Boring Machines (TBM), is conditioned by
a series of factors that reduce, sometimes quite drastically, the
potentiality of the system itself (1), (3), (7), (9). The majority of
these factors affect the theoretically obtainable productivity of the
system in a much more pronounced manner if the altimetric trend of the
tunnel that has to be excavated exceeds such inclinations that it is
necessary to request the use of auxiliary equipment that allows
operation under safe conditions or where it is necessary to back-install
a pre-casted lining that acts as a contrast element and this occurs when
one proceeds in the so-called up-hill excavation (2).

The mean speed of advancement of a TBM is remarkably lower than the
net advancement speed that the machine presents during the excavation
stage (1), (3), (8). The first is usually measured in m/days and from
necessity takes into consideration the stopping times of the machine due
to the installation of the supports, maintenance, the change of tools,
the change of shifts and the waiting times associated to the transport
system of the mucked material.

In short, the factors that can influence the productivity of an
excavation system can be grouped into the following three groups (4),
(5):

* TBM characteristics and its back-up system

* Problems connected to the characteristics of the rock mass which
has to be excavated

* Problems connected to the site organization

The drops in productivity due to the re-gripping that is necessary
with the advancement of the machines and the back-up can be included
within the group inherent to the TBM characteristics and its back-up
system, as can those due to the normal and extraordinary maintenance of
the system and those due to an inadequate "power" of the
excavation machine for the mechanical strength parameters of the rock
mass. For each type of rock there is in fact a critical thrust force on
the tool and an optimal force connected to the lowest waste of specific
energy (Fig. 1), the thrust force should fall between these two values
otherwise the head will work in anomalous conditions that could cause
damage to the tools and slow down the advancement. In the group relative
to the problems arising from the geological and geomechanical structure,
the problems connected to the installation of a support system of the
tunnel can be considered as can those relative to the exchange of the
cutters caused by the wear or support breakage, to the local alteration
and fracturing degree of the rock mass and to the existing
hydro-geological structure (5-7). Finally the problems deriving from the
adopted mucking system, the shift changes of the workers and of the
impositions of a contractual nature, as for example, the carrying out of
investigations at the face during advancement, can all be included in
the group relative to the lowering of productivity connected to the site
organization.

[FIGURE 1 OMITTED]

The set of these factors, some of which are inevitable in that they
are intrinsically necessary to the TBM advancement, can reduce the time
potentially dedicated to excavation to a great extent and therefore also
the efficiency of the system (4).

This study illustrates the results of a comparative analysis
between the productions of tunnels excavated with TBMs with
sub-horizontal axis and those obtained in the case of up-hill tunnels.
The purpose of this comparison is that of supplying indications on the
real productivity of the excavation system using TBMs for altimetric
situations similar to those that have been examined.

MATERIALS AND METHODS

In "normal" altimetric conditions the cycle that
characterizes the mechanized excavation with TBMs basically consists of
two stages: the real excavation, which is possible up to the end of the
jack length and the recall of the machine head support elements and of
the back-up (re-gripping) during which the excavation operations are
interrupted. An exception to this is represented by the double shield
machines for which, if the installation of supports behind the machine
is associated to the excavation operation, the excavation phase can
occur continuously without the re-gripping operations influencing the
production to any great extent.

When up-hill excavating, in the case of open TBMs, it is necessary
to equip the excavation system with means that allows re-gripping of the
machines in safe conditions. This can be obtained by arranging a double
system of grippers. The added grippers, which make up the "fall
preventer device" (Fig. 2), support the entire weight of the
machine and allow the excavation to be performed in safe conditions.
Their action allow the TBM grippers to act as exclusive contrast for the
advancement, as happens for TBMs in excavation operations with
horizontal altimetric axis (Fig. 2).

[FIGURE 2 OMITTED]

A remarkable force should be applied overall from the grippers of
the fall preventer device on the tunnel walls to contrast the weight of
the machine. In some cases, when the rock mass appears fractured with
low persistence discontinuities, problems can occur due to the
detachment of rock blocks from the walls (Fig. 3).

[FIGURE 3 OMITTED]

Another significant difference from the horizontal excavation
system is constituted by the movement of personnel and supply materials.
In up-hill excavation the system is endowed with winches and cable
hauling bogies which are much slower than the vehicles used in
horizontal excavation. On the other hand, the mucking can occur more
quickly as it is possible to arrange the site for the gravity dumping of
the excavated materials in the case of up-hill excavation.

When a collapse occurs, due to the presence of an unforeseen fault
of material of poor geotechnical characteristics, which involves the
area close to the excavation face, the lateral grippers are also given
the task of supporting the weight of the portion of collapsed rock and
the operations of re-establishing stability in the area are more
difficult and longer (Fig. 4) (2).

[FIGURE 4 OMITTED]

RESULTS

The available data relative to production obtained in the
excavation with TBMs of tunnels with diameters between 3.2 and 4.7 m,
with sub-horizontal and up-hill axis, are here given.

Data relative to up-hill excavations: The main characteristics of
the examined cases are shown in Table 1. It can be seen how the open
machines have mainly worked inside massive formations while shielded or
double shielded machines were used for relatively worse formations. From
the qualitative point of view, with reference to the Bieniaski RMR
classification, according to SIA regulations or O-Norm 2203, which was
the reference classifications used in the cited cases, it can be
indicated that the formations crossed by the open TBMs could be
classified as good or very good quality masses (RMR Class I or II and
equivalents) for percentages between 70 and 90%, with uniaxial
compression strength of the rock matrixes that vary from a minimum of 50
MPa for the fine schists to a maximum of 300 MPa for the amphiboles. The
mean of the later falls between 100 and 120 MPa. In the case of shielded
TBMs, no classes of greater reference are available; the uniaxial
compression strength of the matrixes involved in the excavation fall
around 25 MPa. The mean production expressed in metres of excavation,
referring to daytime production (therefore including all the factors
that have constrained the values) are shown in Table 2. The trend of
advancement obtained for the examined cases are shown in Fig. 5, when
available.

[FIGURE 5 OMITTED]

From an examination of the Table 2 it can be noticed how there are
significantly different efficiencies, while, with the exception of two
cases, the mean global production falls around values between 2.5 and
4.9 m [day.sup.-1]. It is also possible to notice that the difference
between the mean daily productions and the maximum ones are higher for
the open machines than for the shielded ones.

This aspect should be compared with the influence that the
characteristics of the rock mass have on the open machines and on the
shielded ones; these last allow productions that are less influenced by
the geomechanical characteristics of the rock mass. It should be
underlined that in the case of Clauson Dixence, the rock mass appears on
average of a lower quality than the mass excavated with open TBMs.

The inclination of the tunnel also appears to play a certain role
in the reduction of the net speed of advancement.

Data relative to horizontal excavations: The main characteristics
of the examined cases are shown in Table 3. These are only limited to
tunnel excavation with open TBMs and slopes for which it has not proved
necessary to make use of machines equipped with fall preventer systems.

Qualitatively speaking, with reference to the previously mentioned
classifications, it can be stated that the formations crossed by the
open TBMs can be classified, with the exception of case 2, as masses of
discrete-good quality (Classes II or III according to the Bieniawski or
equivalent classifications) for overall length percentages of between 60
and 70%, with uniaxial compression strength of the rock matrixes that
vary between a minimum of 20-30 MPa for fine sedimentary rocks to a
maximum of 120-130 MPa for metamorphic rocks.

The mean productions expressed in excavation metres per day,
referring to daytime production, are shown in Table 4. The trend of the
advancement obtained in the examined cases is shown in Fig. 6a, when
available.

[FIGURE 6 OMITTED]

The data shown in the table bring to light the extreme variability
of the global production. From a first examination of the causes of this
variability, a close connection has been deduced between the
characteristics of the excavated rock mass and in particular for the
geomechanical quality of the rock mass and with the presence of poor
rock, from the geomechanical point of view, or of peculiar areas such
as, for example, those with the presence of gas.

The quality of the rock mass, which conditions the installation of
support systems and therefore the consequent stopping times, reduces the
productivity of the system in a proportional manner, while the presence
of particular areas can lower the global production because of stopping
times that are necessary to resolve the problem. It should be underlined
that in the examined cases the stops of this kind were always of a
modest number (usually one single episode, rarely two). As far as the
production is concerned, it has been ascertained that the highest ones
correspond to good-very good quality rock masses found in an almost
uniform manner along the tract, while the lowest production corresponds
to mediocre rock masses associated with one or two stopping episodes to
resolve precise cases.

It could be interesting to compare these productions with those
obtained in three cases of horizontal excavation carried out using
double shielded TBMs, with installation of precasted linings, with
comparable diameters. The characteristic data of these 3 cases and the
obtained productions are shown in Table 5, while the advancement trend
is shown in Fig. 6b.

It should be considered that the Evinos-Mornos project the
excavation was performed inside rock masses prevalently between the
class III and IV and mean uniaxial compression strength of the intact
rock between 60 and 80 MPa, while the T.E.E. project was obtained
excavating a tunnel inside a rock mass of prevalently class II and III,
with mean uniaxial compression strength of the order of 150 MPa.

CONCLUSION

From an examination of the results obtained in the studied cases a
net difference appears evident in terms of productivity between
horizontal and up-hill excavations. The production fields of variability
registered in the examined cases are shown in Fig. 7. AS foreseen, in
the horizontal excavations the variability in the global production is
extremely high. In the examined cases it varies between 7.8 and 40.4 m
[day.sup.-1] and is closely connected to the average geomechanical
quality of the rock mass. Only in some cases did the resolving of an
incident (that is, concentrated stopping time) lead to an important
reduction of the mean global production. In general, the higher limit of
this field of variability is correlated to good-optimal quality rock
masses; on the other hand, the values close to the lower limit are
correlated to rock masses of mediocre quality. Inside this field of
variability, the production obtained with double shielded TBMs,
supported by the installation of precasted linings, can be found, (at
least for the three examined cases) in an intermediate position with
global production between 13.8 and 22.7 m [day.sup.-1]. This allows one
to confirm what has already been indicated by various authors in merit
of a lower susceptibility of the advancement of this kind of TBM to the
quality of the rock mass involved in the excavation. It should be
considered that with the placing of the precast lining behind the
machine, the shielded TBM can supply a final lining and therefore,
strictly speaking, a comparison with open TBMs should be performed also
considering, in the global production, the installation of any final
supports.

[FIGURE 7 OMITTED]

The analysed cases of up-hill excavation highlight a lower
variability of the mean global production. The field of production is
limited on the upper side by a mean production equal to 11 m
[day.sup.-1] and on the lower side by a mean production equal to 2.5 m
[day.sup.-1].

Inside this field of variability it is not possible to distinguish
the benefit of the double shielded TBM on the excavation if not of the
implicit benefit in the case of use of precasted linings.

Such net difference between horizontal and up-hill excavation
productions can be attributed to the greater burdens of times necessary
for the supply of materials and personnel, the regripping operations
with fall preventer systems and, in general, the greater difficulties of
carrying out ordinary and extraordinary maintenance of the machines in
particularly difficult altrimetric conditions. Any incidents,
furthermore, require longer times to resolve, therefore contributing to
a further diminishing of the mean advancement velocity.

(7.) Innaurato, N., C. Oggeri and P.P. Oreste, 2001. Performance
and modeling of indenters and cutting tools for the assessment of the
borability in rock tunneling. Proceeding of the Eurock Symposium Rock
Mechanics, a Challenge for Society, pp: 573-578.